The present invention relates to a brushless DC motor. A conventional brushless DC motor includes: a stator on which drive coils are mounted, a magnetic rotor assembly having magnetic poles arranged alternately in the circumferential direction, a position detector for detecting the position of the rotor relative to the stator, and a drive circuit for applying current to the drive coils according to the detection signal of the position detector. In one example of such a brushless DC motor, as disclosed by Japanese Laid-Open Patent Application No. 111689/1980, a rotary encoder coupled to the magnetic rotor is employed as the position detector. Application of current to the drive coils is controlled according to the output signal of the rotary encoder.
The conventional brushless DC motor suffers from the following difficulties: In the case of two-phase drive coils, it is necessary to provide two position detectors for detection of the rotor position, in the case of three-phase drive coils, it is necessary to provide three position detectors, etc. That is, the motor requires a plurality of position detectors, and thus is intricate in its physical and electrical arrangement. The position detection signals are, in general, converted into switching waveforms which are applied to the drive coils without modification. As such, the output torque characteristic of the motor includes harmonic components, which makes the motor vibrate and produce noise. No matter what steps are taken to supply drive signals to the coils which are substantially in the form of a sine wave, it is impossible to obtain signals having a sufficiently high waveform accuracy because of differences in sensitivity between ones of the position detectors, or because of sensitivity variations such as may be due to temperature variations. Accordingly, for this reason also, such a prior art motor is not acceptable.